The R2CW is a VHF CW/SSB transceiver with outstanding performance. This makes it ideal for contest use. A drawback of this transceiver is that it lacks some of the features found in modern equipment. One such feature is a visual signal display. However, an SDR spectrum analyzer addresses this need. The circuit and concept are, in principle, suitable for other transceivers as well, not just the R2CW.
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R2CW Intermediate Frequency Connection Point
The R2CW uses a single 9 MHz intermediate frequency (IF). An 8-crystal SSB filter is connected to this IF. If we want to cover the entire signal spectrum, we must tap the IF signal before this filter. A possible point is the collector (KFW16A) of transistor T22 on the mixer board (SMS), as shown in the diagram:
Intermediate Frequency Connection Diagram for the R2CW Transceiver
An isolation amplifier is connected to this point. Its purpose is to connect the SDR receiver in such a way that the signal is not affected and no unwanted signals leak from the SDR receiver.
G4HUP HupRF Isolation Amplifier
The circuit for this isolation amplifier is well known. It utilizes the high input impedance of the J310 FET, supplemented by an emitter follower with a second transistor. This circuit is characterized by a relatively low output impedance, which allows for connection to the input of an SDR receiver.
The circuit is built on a piece of universal printed circuit board. There are no hidden complications, but care must be taken when soldering the FET transistor (due to its sensitivity to electromagnetic fields). The circuit board is housed in a tin-plated sheet metal box.
Connection in the R2CW Transceiver
The transceiver must be desoldered. The mixer board (SMS) must be removed, which involves desoldering the box’s ground connection to this circuit and pulling it out of the backplane. After opening the enclosure, locate the collector of transistor T22 and route the signal out of it using a thin coaxial cable. Then close the enclosure and reconnect it.
SMA Output Connector on the R2CW Rear Panel
The box containing the isolating amplifier is soldered to the ground plane of the R2CW main board. A hole for the SMA connector is drilled in the rear panel.
Conclusion
Adding an SDR spectrum analyzer to the R2CW makes it a slightly more modern device, and the effort is well worth it. It is also possible to use a cheaper Softrock-type SDR receiver or an SDR receiver in the form of a USB flash drive.
Recommended Resources
https://sites.google.com/site/om1aeg/bastlenie-projekty/sdr-panadapter-ft857
http://huprf.com/huprf/pat-board/